I've created new patch where I've added new option called
skip-compile-dir. Patch in attachment. I didn't make ChangeLog yet.
If you like this implementation I prepare ChangeLog and testsuite script.
I can change something if you want.

Here is example:

$ gcc -g -Wall /home/eldar/testdir/test.c
$ ./contrib/gdb-7.2/gdb/gdb ./a.out
...
(gdb) b main
...
(gdb) r
...
(gdb) backtrace
#0  main () at /home/eldar/testdir/test.c:4
...
(gdb) set backtrace
List of set backtrace subcommands:
...
set backtrace skip-compile-dir -- Set whether compile path should be
skipped in backtraces
...
(gdb) set backtrace skip-compile-dir on
(gdb) backtrace
#0  main () at testdir/test.c:4
(gdb) show backtrace
...
skip-compile-dir:  Whether compile path should be skipped in backtraces is on.
(gdb) show backtrace skip-compile-dir
Whether compile path should be skipped in backtraces is on.